Antimatter

August sees the 80th anniversary of Carl Anderson’s discovery of the positron, the antimatter counterpart to the electron. But what is antimatter, how can it be used – and is it dangerous?

The existence of antimatter was first predicted by Paul Dirac in papers published from 1928 onwards.

Classical physics only allowed systems to have positive energy. But Dirac’s new theory of relativistic quantum mechanics allowed for a particle with negative energy solution, as a counterpart to the familiar positive-energy electron.

After ruling out the possibility that this particle was simply the proton – which has a hugely greater mass – Dirac predicted the existence of a new particle with the same mass of the electron but with a charge that was positive rather than negative.

That particle was found experimentally on 2 August 1930. Carl Anderson was observing the trails produced in the particle shower that was created in his cloud chamber when cosmic rays passed through it. His observations included a particle with the same mass as the electron but the opposite charge – its track bent in the “wrong” direction in a magnetic field. Anderson coined the name “positron” for his new discovery.

In 1933 Dirac went on to predict the existence of the antiproton, the counterpart to the proton. It was discovered in 1955 by Emilio Segrè and Owen Chamberlain at the University of California, Berkeley.

It’s now understood that all particles have an equivalent antimatter particle with opposite charge and quantum spin – although some are their own antiparticle. However hardly any antimatter is seen in the observable universe, and why there should be vastly much more normal matter is one of the great unsolved problems in physics.

Creation and destructionIt was once thought that matter could neither be created nor destroyed, but we now know that energy and mass are interchangeable. When a particle collides with its antiparticle the two annihilate each other, with their mass being entirely converted into energy.

That energy creates a shower of new particles, which serve as a hint that such an event has taken place – for example, detecting a gamma ray with an energy of 511 keV is a signature of an electron and a positron annihilating one another.

Antiparticles can be created either naturally or artificially.

Positrons are commonly produced by radioactivity – they’re a byproduct of β+ decay, in which a proton in the atomic nucleus transmutes into a neutron.

Other antiparticles result from high-energy collisions, in which the excess energy produces pairs of particles and their antimatter counterparts.

This process can be harnessed to produce antimatter artificially by, for example, colliding a stream of high-energy protons with a dense target in order to produce antiprotons.

Although it’s also possible to make whole atoms from antimatter, because they have no net charge they can’t be stored magnetically like positrons and antiprotons can, and risk annihilating with any container.

Application and speculationAntimatter annihilations convert the entire mass of the particles involved into energy, following Albert Einstein’s famous equation E = mc2.

A great deal of energy can be produced from little mass – a kilogram of matter annihilating with the same amount of antimatter will release around as much as the Tsar Bomba, the largest thermonuclear bomb ever built.

Because of this, antimatter has been touted as a possible future weapon or source of fuel – antimatter-driven propulsion is a staple of science fiction.

However, antimatter currently takes far too long to produce, and at too high an energy cost, for either weapons or fuel to be practicable. CERN claims it has taken several hundred million pounds to produce just a billionth of a gram, and that to make a gram of antimatter would take about a 100 billion years.

And yet antimatter does have some important uses.

One type of medical scan, Positron Emission Tomography, utilises radioactive ‘tracers’ that undergo β+ decay. When the tracers emit a positron, it collides with an electron in the body and the resultant annihilation event produces a pair of gamma rays.

Detecting those gamma rays allows medical staff to build a picture of the concentration of the tracer throughout the patient’s body. Commonly the tracer used is a glucose analogue, which is taken up in high quantities by the brain, the liver and most cancers – allowing the detection of tumours.

It’s also been suggested that antimatter can be used not only to diagnose cancer but also to treat it, using a technique similar to ion therapy.

This uses a beam of protons to irradiate, and therefore destroy, a tumour without affecting the surrounding tissue, which the beam simply passes through. It’s possible that if antiprotons are used instead, extra energy would be deposited around the tumour when it annihilates with a normal-matter particle within the body, giving it two blasts instead of just one – antimatter potentially saving lives a few decades after it was first discovered.